Triangular-wave controlled amplitude-modulated flow analysis for extending dynamic range to saturated signals.

We have applied our amplitude-modulated flow analysis concept to extend the dynamic range to saturated analytical signals. Sample solution, the flow rate FS of which is periodically varied with a triangular control signal Vc, is merged with a reagent solution delivered at a constant flow rate FR. While the total flow rate FT is kept constant, a diluent with FT-(FS + FR) flow rate is aspirated from the third channel. The analytical signal Vd obtained downstream is processed by fast Fourier transform to obtain the amplitudes of the wave components in Vd. When the sample concentration CS is low, Vd shows a triangular profile like Vc; the sum of the amplitudes ΣA is proportional to CS. When CS is high, Vd shows a trapezoidal profile because of the Vd saturation. A linear calibration curve can also be obtained for such saturated signals by plotting CSΣA against CS. The proof of the concept is validated by applying it to spectrophotometric determinations of a dye (Methylene Blue) and colored complexes of Fe2+ - phenanthroline and Fe3+ - Tiron.